Abstract

For many cases modeled and measured UV global irradiances agree to
within ±5% for cloudless conditions, provided that all relevant
parameters for describing the atmosphere and the surface are well
known. However, for conditions with snow-covered surfaces this
agreement is usually not achievable, because on the one hand the
regional albedo, which has to be used in a model, is only rarely
available and on the other hand UV irradiance alters with different
snow cover of the surface by as much as 50%. Therefore a method is
given to determine the regional albedo values for conditions with snow
cover by use of a parameterization on the basis of snow depth and snow
age, routinely monitored by the weather services. An algorithm is
evolved by multiple linear regression between the snow data and
snow-albedo values in the UV, which are determined from a best fit of
modeled and measured UV irradiances for an alpine site in
Europe. The resulting regional albedo values in the case of snow
are in the 0.18–0.5 range. Since the constants of the regression
depend on the area conditions, they have to be adapted if the method is
applied for other sites. Using the algorithm for actual cases with
different snow conditions improves the accuracy of modeled UV
irradiances considerably. Compared with the use of an average,
constant snow albedo, the use of actual albedo values, provided by the
algorithm, halves the average deviations between measured and modeled
UV global irradiances.

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